In devices such as a NAND flash memory device having a floating gate structure as a data storage element, a control gate electrode is typically formed above the floating gate electrode via an interelectrode insulating film in a memory-cell transistor.
In such case, a select transistor and a peripheral circuit transistor formed simultaneously with a memory-cell transistor is not needed to store the data. Thus, an opening is formed through the interelectrode insulating film to electrically connect the overlying electrode film and the underlying electrode film.
Such structure is formed by the following manufacturing process flow. A lower electrode film, an interelectrode insulating film, and a thin layer of first upper electrode film are stacked, and an opening of a predetermined width is formed by etching through the first upper electrode film and the interelectrode insulating film in portions where the gate electrodes are to be formed. The etching further progresses into the underlying lower electrode film to ensure that the upper and the lower electrode films are connected. Thus, in the opening formed through the first upper electrode film and the interelectrode film, and in the recess formed into the over-etched lower electrode film, a step is formed. Then, a second upper electrode film is filled into the opening and the recess to obtain an electrically connected structure.
In such structure, it may not be possible to eliminate the step depending upon the relation between the width of the opening and the thickness of the upper electrode film filing. When the upper electrode film is thin, above phenomenon easily occurs.
For instance, in some NAND flash memory devices, control gate electrode has a metal film formed above the upper surface of a thin electrode film comprising a silicon film via a barrier metal film. The presence of a step in such structure may cause the barrier metal film to be thinned or disconnected on the step. As a result the metal film stacked after the formation of the barrier metal film may come in direct contact with the underlying silicon film and may cause unwanted failures.